The invention relates to resorbable bone implant material prepared from a powdery component containing hydroxyl apatite and a liquid, as well as a method for preparing the same.
The importance of bone replacement materials, in particular in the areas of orthopedics, traumatology, cranial, dental and facial surgery, and orthodontics is still increasing. Significant areas of applicationxe2x80x94both in human medicine as in veterinary medicinexe2x80x94are, for example, the closing of large bone defects associated with comminuted fractures as well as the attachment of small bone fragments, the filling of bone defects resulting from bone cysts and after removal of bone tumors, the filling of voids caused by chronic osteomyelitis, applications associated with material loss on alveolis and jaw bones and the use as a carrier material, for example, for antibiotics, cytostatic, and osteogenic materials. The number of applications of bone replacement materials which increases worldwide year-to-year relates directly to their broad areas of indications. Several reasons therefor are the increasing life expectancy, the increasing industrialization, the increasing traffic density and steadily improving surgical techniques which allow any increasingly broader use of these materials. The need for bone replacement materials is so great because the materials which are best suited, namely autogenic spongy bone, are available only in limited quantities. In addition, their use requires a second, additional surgical procedure subject to the entire range of medical-surgical risks of any surgical procedure, whereby the newly created defects must in most cases be filled in againxe2x80x94with a bone replacement material.
Allogenic bonesxe2x80x94which represent the next best implant materialxe2x80x94can almost never be used today due to the high-risk for transmission of infectious substances, such as hepatitis and HIV virus, or of the Kreutzfeld-Jacob disease. Moreover, providing an equivalent tested and safe material, as well as operating bone banks require financial outlays that are often beyond reach.
Bone replacement materials which satisfy the stringent biological and chemical-physical requirements, are hence a medical and economical necessity and the development goal of several research groups.
Among the many implant materials that have been developed over the years, there were several that have found widespread application and which satisfy more or less today""s medical and regulatory requirements for medical products. However, these products do not always meet the requirements expected from such medical products, namely availability, purity, reproducibility, standardizability, physical-chemical stability or compatibility. More frequently used today are, in particular, hydroxyl apatite ceramics, which are materials that are not broken down in the body, and calcium sulfate (xe2x80x9cPlaster of Parisxe2x80x9d, hemihydrate of calcium sulfate) belonging to the group of resorbable bone replacement materials.
Hydroxyl apatite which can be implanted both in compact and in porous form, as a solid material or as granules or powder, is practically not broken down and therefore remains in the organism permanently and almost unchanged. With respect to integration with the bone, porous implants with an interconnecting system of voids which, if implanted with a pass fit in the form of blocks or cylinders, achieve the best results by forming an intimate connection with the host bone over the largest possible area. Corresponding animal experiments have shown that within several months, the implants that fill a bone defect are infused by newly formed bone that originates from the contact surfaces and covers the surface of the system of pores like a wallpaper cover.
The significant disadvantage of hydroxyl apatite is its inherent brittleness, so that implants of this type never reach the mechanical strengthxe2x80x94in particular the elasticityxe2x80x94of the surrounding host bone, which severely limits its medical applications. Although it has been shown that porous hydroxyl apatite cylinders became mechanically stronger after the healing process than in their initial form, it was simultaneously observed that the webs between the pores had fissures and gaps, which could again indicate a weakening of the implant. Since these types of implants remain in the organism permanently, they also represent in the long run defects which can lead to a permanent weakening of the host bone and a permanent fracture risk, in particular in bone regions that are mechanically severely stressed.
On the other hand, the material hydroxide apatite itself biologically interacts with the host tissue in a rather positive way.
To get around the disadvantages of a non-resorbable bone replacement that remains intact even after complete integration into the bone, research focused during recent years increasingly on implant materials that can be broken down. In this field, in particular calcium sulfate has found a renaissance, considering that aside from several more recent publications, a first report appeared in 1892, wherein xe2x80x9cPlaster of Parisxe2x80x9d was used for filling of tuberculous and osteomyelitic bone defects.
Like hydroxyl apatite, calcium sulfate has advantageous biological properties. However, unlike hydroxyl apatite, depending on its shape and volume, calcium sulfate is very quickly broken down and resorbed in the organism within several weeks or months.
This actually desirable and advantageous feature is negated in that the implant tends to be broken down significantly faster than the bone can regrow from the implant bed. This causes voids to be formed once again which are then typically no longer filled by bone, but rather by connecting tissue.
WO 87/05521 A1 describes a bone implant material in form of a plastic, moldable mass for filling bone defects. The mass consists of sintered hydroxyl apatite granules with a grain size of 250-5000 xcexcm, which is substantially insoluble and/or cannot be broken down in the organism, and calcium sulfate-hemihydrate. The materials described above are commercially available, but their manufacturing conditions, physical-chemical and medical properties and in particular their purity cannot be ascertained. The dry components are mixed together in the ratio of 70-60% to 30-40% and then mixed further with a suitable liquid (water, physiological sodium chloride solution) into a moldable paste which solidifies in situ after application. According to the description, the calcium sulfate dissolves quickly in the bone defect (within several days to several weeks). The voids which are formed between the hydroxyl apatite granules that are situated more or less loosely in the defect, should then enable the newly formed bone to grow into the voids. Due to accepted medical understanding of the physiological processes during bone healing, the aforedescribed product as well as the way in which it is applied appear to have problems in several aspects. The rapid dissolution of calcium sulfate, which has been promoted as an advantage, induces the risk that the calcium sulfate is dissolved faster than new bone can grow. In this case, connective tissue could grow around the hydroxyl apatite granules which are not connected to each other and therefore do not provide mechanical stability as an implant material, so that the defect would not be filled by bone. This risk also seems to manifest itself in a decrease of the compressive strength of the fill material by 53% after only two days in an in vivo experiment conducted in sodium chloride solution. The results of the described animal experiments are not convincing. For example, it was mentioned that xe2x80x9csmall defects (small holes)xe2x80x9d occur in the rabbit tibia; these, however, are known to be filled by bone during the physiological repair processes without requiring additional filling. Also, the reported defects after tooth extraction on beagles were of small size. The defects were in both cases overgrown by gingiva after 7-10 days, with and without implant.
Two patents (EP 0 159 087 A1 and EP 0 159 089 A1) describe resorbable implant materials used as carriers of active ingredients for releasing drugs in bone or tissue. The materials are each composed of two components. In the first case, calcium sulfate and calcium carbonate powder in a ratio of 0.3 to 1.0 is mixed with a drug and thereafter mixed with water to form a paste. After the mass has solidified, it can be introduced into the organism, for example, as a carrier for antibiotics. By using calcium carbonate, the implant is broken down much more slowly than plaster (within several months), which should leave sufficient time for the healing process, i.e., to replace the implant, for example in a bone defect, with newly formed bone. In the second case, calcium sulfate is mixed with calcium mono-hydrogen phosphate and the dried mass is subsequently heated to 700xc2x0 C. until the calcium hydrogen phosphate melts. The resulting micro-porous mass can be impregnated with the drugs and placed in the organism as a drug carrier. However, the chemical-physical and/or medical properties of the aforementioned powdery components were not specified.
Finally, the patent application GB 2 323 083 A describes the manufacture of an implant material for filling bone defects, wherein the implant material is composed of a mixture of sintered or non-sintered hydroxyl apatite granules with a grain size of 100-400-5000 xcexcm and hydroxyl apatite powder with a grain size of 1-40 xcexcm in a ratio of 1:0.1 to 1:1. This mixture is heated to 800-1200xc2x0 C. either dry or as an aqueous suspension, optionally by adding dispersion promoters or binders. The fine powder connects the larger granules with each other during the sintering process, so that solid form bodies (blocks) can be produced, which can then be used for filling bone defects. Since irregular bone defects cannot be formfittingly filled with blocks, the blocks are crushed by applying finger pressure during application and thereby separated into the initial granules, which are then used to fill the defects. When present as sintered hydroxyl apatite, the implant material cannot be broken down in the organism. Based on current medical understanding, problems can arise when filling bone defects with separate larger granules due to the mechanical instability of the implant material. Permeation of bone into the implant is also doubtful, so that the method described in the above referenced patent appears to be without a logical foundation. The question has to be asked why solid form bodies, which by themselves are poorly suited for filling defects, are initially produced from granules by a complex production process, only to be laterxe2x80x94just to compensate for this disadvantagexe2x80x94destroyed during application by producing and implanting again the initial granules.
It is an object of the invention to provide a resorbable implant material, in particular for use in surgery, orthopedics, traumatology and dentistry, which combines the advantages of hydroxyl apatite and calcium sulfate and simultaneously eliminates the associated disadvantages, as well as a method for producing such implant material.